Image forming apparatus

ABSTRACT

An image forming apparatus includes a photoconductor, an optical scanner, a development device, a movable density sensor, a density sensor driver, and a processor. The optical scanner includes a light source to emit light, and irradiates a surface of the photoconductor in a main scanning direction with the light to form a latent image on the surface of the photoconductor. The development device develops the latent image into a toner image. The density sensor detects unevenness in density of the toner image in the main scanning direction. The density sensor driver moves the density sensor in the main scanning direction. The processor corrects a driving signal for the light source according to image data to reduce the unevenness in density in the main scanning direction, according to positional data of the density sensor in the main scanning direction and an output value of the density sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35U.S.C. §119(a) to Japanese Patent Application No. 2013-216475, filed onOct. 17, 2013, in the Japan Patent Office, the entire disclosure ofwhich is hereby incorporated by reference herein.

BACKGROUND

1. Technical Field

Embodiments of the present invention generally relate to an imageforming apparatus, and more specifically, to an image forming apparatususing laser light.

2. Background Art

Various types of electrophotographic image forming apparatuses areknown, including copiers, printers, facsimile machines, or multifunctionmachines having two or more of copying, printing, scanning, facsimile,plotter, and other capabilities. Such image forming apparatuses usuallyform an image on a recording medium according to image data.Specifically, in such image forming apparatuses, for example, a chargeruniformly charges a surface of a photoconductor serving as an imagecarrier. An optical writer irradiates the surface of the photoconductorthus charged with a light beam to form an electrostatic latent image onthe surface of the photoconductor according to the image data. Adevelopment device supplies toner to the electrostatic latent image thusformed to render the electrostatic latent image visible as a tonerimage. The toner image is then transferred onto a recording mediumdirectly or indirectly via an intermediate transfer belt. Finally, afixing device applies heat and pressure to the recording medium carryingthe toner image to fix the toner image onto the recording medium. Thus,the image is formed on the recording medium.

SUMMARY

In one embodiment of this disclosure, an improved image formingapparatus is described that includes a photoconductor, an opticalscanner, a development device, a movable first density sensor, a densitysensor driver, and a processor. The optical scanner includes a lightsource to emit light, and irradiates a surface of the photoconductor ina main scanning direction with the light to form a latent image on thesurface of the photoconductor. The development device develops thelatent image into a toner image. The movable first density sensordetects unevenness in density of the toner image in the main scanningdirection. The density sensor driver moves the first density sensor inthe main scanning direction. The processor corrects a driving signal forthe light source according to image data to reduce the unevenness indensity in the main scanning direction, according to positional data ofthe first density sensor in the main scanning direction and an outputvalue of the first density sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be more readily obtained as the same becomesbetter understood by reference to the following detailed description ofembodiments when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic view of an image forming apparatus according to anembodiment of the present invention;

FIG. 2 is a schematic view of an imaging unit incorporated in the imageforming apparatus of FIG. 1;

FIG. 3A is a schematic view of a secondary transfer portion of the imageforming apparatus of FIG. 1 upon printing;

FIG. 3B is a schematic view of the secondary transfer portion of theimage forming apparatus of FIG. 1 upon measurement of a referencepattern;

FIG. 4 is a schematic view of a density sensor drive mechanism accordingto a first embodiment incorporated in the image forming apparatus ofFIG. 1;

FIG. 5A is a schematic view of a density sensor incorporable in theimage forming apparatus of FIG. 1;

FIG. 5B is a schematic view of an alternative density sensor;

FIG. 5C is a schematic view of another alternative density sensor;

FIG. 6 is a block diagram of a processor incorporated in the imageforming apparatus of FIG. 1;

FIG. 7 is a flowchart of a shading correction process according to thefirst embodiment;

FIG. 8A is a plan view of the density sensor drive mechanism with areference pattern;

FIG. 8B is a graph of density sensor output;

FIG. 8C is a graph of an optical scanner driving signal;

FIG. 9 shows a layout of a plurality of density sensors for adjustingimage density;

FIG. 10 is a schematic view of a density sensor drive mechanismaccording to a second embodiment;

FIG. 11 is a schematic view of a density sensor drive mechanismaccording to a third embodiment;

FIG. 12 is a schematic view of a density sensor drive mechanismaccording to a fourth embodiment;

FIG. 13A is a plan view of a density sensor drive mechanism according toa fifth embodiment;

FIG. 13B is a graph of density sensor output according to the fifthembodiment;

FIG. 14 is a plan view of a density sensor drive mechanism according toa sixth embodiment;

FIG. 15 is a flowchart of a shading correction process according to thesixth embodiment;

FIG. 16 is a plan view of an image carrier illustrating positions todetect density according to a seventh embodiment;

FIG. 17A is an overall flowchart of a print job including determinationof whether or not to perform a shading correction process according tothe seventh embodiment;

FIG. 17B is a flowchart of the shading correction process illustrated inFIG. 17A;

FIG. 18 is a diagram illustrating a comparative shading correction in amain scanning direction; and

FIG. 19 is a graph of image density illustrating a problem of thecomparative shading correction of FIG. 18.

The accompanying drawings are intended to depict embodiments of thepresent invention and should not be interpreted to limit the scopethereof The accompanying drawings are not to be considered as drawn toscale unless explicitly noted.

DETAILED DESCRIPTION

In describing embodiments illustrated in the drawings, specificterminology is employed for the sake of clarity. However, the disclosureof this patent specification is not intended to be limited to thespecific terminology so selected and it is to be understood that eachspecific element includes all technical equivalents that have the samefunction, operate in a similar manner, and achieve similar results.

Although the embodiments are described with technical limitations withreference to the attached drawings, such description is not intended tolimit the scope of the invention and all of the components or elementsdescribed in the embodiments of the present invention are notnecessarily indispensable to the present invention.

In a later-described comparative example, embodiment, and exemplaryvariation, for the sake of simplicity like reference numerals are givento identical or corresponding constituent elements such as parts andmaterials having the same functions, and redundant descriptions thereofare omitted unless otherwise required.

It is to be noted that, in the following description, suffixes Y, M, C,and K denote colors yellow, magenta, cyan, and black, respectively. Tosimplify the description, these suffixes are omitted unless necessary.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views,embodiments of the present invention are described below.

According to an embodiment of the present invention, an image formingapparatus drives a density sensor for shading correction in a mainscanning direction with a density sensor drive mechanism. By measuringdensity for shading correction while moving the density sensor in themain scanning direction, the image forming apparatus can continuouslymeasure density data at a plurality of positions on an image carrierwith the same sensor. Accordingly, the single sensor enhances shadingcorrection at high resolution and low production costs, withoutrequiring interpolation such as linear interpolation.

Initially with reference to FIG. 1, a description is given of an overallconfiguration and operation of an image forming apparatus 1 according toan embodiment of the present invention.

FIG. 1 is a schematic view of the image forming apparatus 1. In thepresent embodiment, the image forming apparatus 1 is a color laserprinter.

The image forming apparatus 1 includes four centrally located imagingunits 4Y, 4M, 4C, and 4K. The imaging units 4Y, 4M, 4C, and 4K areidentical in configuration, except that they accommodate developers ofdifferent colors. Specifically, the imaging units 4Y, 4M, 4C, and 4Kaccommodate developers of yellow (Y), magenta (M), cyan (C), and black(K), respectively, which are used to form a color toner image.

Each of the imaging units 4Y, 4M, 4C, and 4K includes, e.g., adrum-shaped photoconductor 5 serving as an image carrier that carries anelectrostatic latent image and a resultant toner image, a charger 6 thatcharges an outer circumferential surface of the photoconductor 5, adevelopment device 7 that supplies toner to the electrostatic latentimage formed on the outer circumferential surface of the photoconductor5 to render the electrostatic latent image visible as a toner image, anda cleaner 8 that cleans the outer circumferential surface of thephotoconductor 5. It is to be noted that, in FIG. 1, reference numeralsare assigned to the photoconductor 5, the charger 6, the developmentdevice 7, and the cleaner 8 of the imaging unit 4K that forms a blacktoner image, whereas reference numerals for the imaging units 4Y, 4M,and 4C that form yellow, magenta, and cyan toner images, respectively,are omitted.

An exposure device 9 serving as an optical scanner is disposed below theimaging units 4Y, 4M, 4C, and 4K. The exposure device 9 includes, e.g.,a light source to emit a laser light beam Lb, a polygon mirror topolarize the laser light beam Lb, and a scanning optical systemincluding an f-θ lens and reflection mirrors to direct the laser lightbeam Lb to the outer circumferential surfaces of the photoconductors 5.Thus, the exposure device 9 irradiates the outer circumferentialsurfaces of the respective photoconductors 5 with the laser light beamLb according to image data.

As the polygon mirror rotates, it moves an optical spot formed on theouter circumferential surface of each photoconductor 5 axially along thephotoconductor 5. Thus, one-line scanning is performed. Upon completionof the one-line scanning, the photoconductor 5 is rotated for a nextscanning. It is to be noted that the axial direction of thephotoconductors 5 is referred to as the main scanning direction, and arotational direction of the photoconductors 5 is referred to as thesub-scanning direction. The position of the optical spot on the outercircumferential surface of each photoconductor 5 in the main scanningdirection is referred to as the image height.

A transfer device 3 is disposed above the imaging units 4Y, 4M, 4C, and4K. The transfer device 3 includes, e.g., an intermediate transfer belt30 serving as an intermediate transfer body, four primary transferrollers 31 serving as primary transfer devices, a secondary transferroller 36 serving as a secondary transfer device, a secondary transferbackup roller 32, a cleaning backup roller 33, a tension roller 34, anda belt cleaner 35.

The intermediate transfer belt 30 is an endless belt stretched aroundthe secondary transfer backup roller 32, the cleaning backup roller 33,and the tension roller 34. In the present embodiment, the secondarytransfer backup roller 32 is rotated to rotate the intermediate transferbelt 30 in a direction indicated by arrow X in FIG. 1.

The intermediate transfer belt 30 is sandwiched between the four primarytransfer rollers 31 and the respective photoconductors 5. Thus, fourprimary transfer areas herein called primary transfer nips N1 are formedbetween the intermediate transfer belt 30 and the photoconductors 5. Theprimary transfer rollers 31 are connected to a power supply that appliesa predetermined direct current (DC) voltage and/or alternating current(AC) voltage to each of the primary transfer rollers 31.

The intermediate transfer belt 30 is also sandwiched between thesecondary transfer roller 36 and the secondary transfer backup roller32. Thus, a secondary transfer area herein called a secondary transfernip N2 is formed between the secondary transfer roller 36 and theintermediate transfer belt 30. Similar to the primary transfer rollers31, the secondary transfer roller 36 is connected to the power supplythat applies a predetermined direct current (DC) voltage and/oralternating current (AC) voltage to the secondary transfer roller 36.

The belt cleaner 35 includes a cleaning brush and a cleaning blade thatcontact an outer circumferential surface of the intermediate transferbelt 30. A waste toner conveyance tube extending from the belt cleaner35 to an intake of a waste toner container conveys waste toner collectedfrom the intermediate transfer belt 30 by the belt cleaner 35 to thewaste toner container. A density sensor 50, described later, is disposedwhere the density sensor 50 can measure the outer circumferentialsurface of the intermediate transfer belt 30, with, e.g., supportmembers and a density sensor driver described later.

A bottle holder 2 is disposed in an upper portion of the image formingapparatus 1. Toner bottles 2Y, 2M, 2C, and 2K are detachably attached tothe bottle holder 2 to contain fresh toner of yellow, magenta, cyan, andblack, respectively. The fresh toner is supplied from the toner bottles2Y, 2M, 2C, and 2K to the respective development devices 7 through tonersupply tubes connecting the toner bottles 2Y, 2M, 2C, and 2K with therespective development devices 7.

A tray 10 is disposed in a lower portion of the image forming apparatus1. The tray 10 accommodates recording media such as a plurality ofsheets P of plain paper. Alternatively, the recording media may be,e.g., postcards, envelopes, overhead projector (OHP) transparencies, orsheets of thick paper, thin paper, coated paper, art paper, or tracingpaper. A feed roller 11 is also disposed in the lower portion of theimage forming apparatus 1. The feed roller 11 feeds a sheet P from thetray 10 toward the secondary transfer nip N2 formed between thesecondary transfer roller 36 and the intermediate transfer belt 30.Additionally, a bypass tray may be attached to the image formingapparatus 1 to place such recording media thereon.

A conveyance passage R extends from the feed roller 11 to a pair ofdischarge rollers 13 to convey the sheet P from the tray 10 to the pairof discharge rollers 13 through the secondary transfer nip N2, andconsequently, out of the image forming apparatus 1. In the conveyancepassage R, a pair of timing rollers 12 is disposed upstream from thesecondary transfer nip N2 in a direction in which the sheet P isconveyed (hereinafter simply referred to as sheet conveying direction).The pair of timing rollers 12 sends out the sheet P fed from the feedroller 11 toward the secondary transfer nip N2 at a predetermined time.

A fixing device 20 is disposed downstream from the secondary transfernip N2 in the sheet conveying direction in the conveyance passage R. Atoner image transferred from the intermediate transfer belt 30 onto thesheet P at the secondary transfer nip N2 is fixed on the sheet P in thefixing device 20. The pair of discharge rollers 13 is disposeddownstream from the fixing device 20 in the sheet conveying direction inthe conveyance passage R. The pair of discharge rollers 13 dischargesthe sheet P carrying the fixed toner image outside the image formingapparatus 1, specifically onto a discharge tray 14 disposed atop theimage forming apparatus 1. The output tray 14 stocks the sheet Pdischarged by the pair of discharge rollers 13.

With continued reference to FIG. 1, a description is now given of abasic image forming operation of the image forming apparatus 1.

When a print job starts, the respective photoconductors 5 of the imagingunits 4Y, 4M, 4C, and 4K are rotated in a clockwise direction in FIG. 1.The chargers 6 uniformly charge the outer circumferential surfaces ofthe photoconductors 5 with an electrical charge of a predeterminedpolarity. The exposure device 9 irradiates the outer circumferentialsurfaces of the photoconductors 5 thus charged with the laser light beamLb according to yellow, magenta, cyan, and black image data constructingimage data of a desired full-color image to form electrostatic latentimages on the outer circumferential surfaces of the photoconductors 5,respectively. The development devices 7 supply toner to theelectrostatic latent images thus formed on the photoconductors 5 torender the electrostatic latent images visible as yellow, magenta, cyan,and black toner images, respectively.

Simultaneously, when the print job starts, the secondary transfer backuproller 32 is rotated in a counterclockwise direction in FIG. 1, therebyrotating the intermediate transfer belt 30 in the direction indicated byarrow X. The power supply applies a constant voltage or a constantcurrent control voltage having a polarity opposite a polarity of thetoner to the primary transfer rollers 31 to generate a transfer electricfield at each primary transfer nip N1 formed between the photoconductor5 and the corresponding primary transfer roller 31.

When the yellow, magenta, cyan, and black toner images formed on thephotoconductors 5 reach the primary transfer nips N1, respectively, inaccordance with rotation of the photoconductors 5, the transfer electricfields generated at the primary transfer nips N1 transfer the yellow,magenta, cyan, and black toner images from the photoconductors 5 ontothe intermediate transfer belt 30, respectively, such that the yellow,magenta, cyan, and black toner images are superimposed successively onthe intermediate transfer belt 30. Thus, a full-color toner image isformed on the outer circumferential surface of the intermediate transferbelt 30.

In the following description, the term “image carrier(s)” is used torepresent the photoconductors 5 and the intermediate transfer belt 30.The image carrier carries a toner image formed thereon by developing anelectrostatic latent image formed by an optical scanning system. Forexample, if an image forming apparatus transfers a toner image onto aprinting material such as a sheet of paper from a photoconductor, thephotoconductor is an image carrier. By contrast, if an image formingapparatus transfers a toner image onto printing material from aphotoconductor via a transfer belt, the photoconductor and the transferbelt are image carriers.

Referring now to FIG. 2, a description is given of the imaging units 4.

FIG. 2 is a schematic view of one of the imaging units 4 with associatecomponents.

Each of the photoconductors 5 is surrounded by the cleaner 8, a chargingroller as the charger 6, a mirror 9 a of the exposure device 9, and thedevelopment device 7, disposed upstream to downstream, in that order, ina direction indicated by arrow Y. The development device 7 includes adevelopment roller 7 a. The development roller 7 a transfers the tonerimage formed on the photoconductor 5 onto the intermediate transfer belt30 at the primary transfer nip N1 formed between the photoconductor 5and the primary transfer roller 31. The cleaner 8 includes a cleaningblade 8 a. After the primary transfer of the toner image from thephotoconductor 5 onto the intermediate transfer belt 30, the cleaningblade 8 a removes residual toner that failed to be transferred onto theintermediate transfer belt 30 and therefore remaining on thephotoconductor 5 from the photoconductor 5. Thereafter, a dischargerremoves the charge on the outer circumferential surface of thephotoconductor 5 to ready the photoconductor 5 for the next imageformation.

Referring back to FIG. 1, the feed roller 11 disposed in the lowerportion of the image forming apparatus 1 is rotated to feed a sheet Pfrom the tray 10 toward the pair of timing rollers 12 in the conveyancepassage R. When the sheet P comes into contact with the pair of timingrollers 12, the pair of timing rollers 12 temporarily stops conveyanceof the sheet P.

Thereafter, the pair of timing rollers 12 is rotated at a predeterminedtime to convey the sheet P to the secondary transfer nip N2 insynchronization with the full-color toner image formed on theintermediate transfer belt 30 reaching the secondary transfer nip N2. Atthis time, a transfer voltage having a polarity opposite a polarity ofthe charged toner contained in the full-color toner image formed on theintermediate transfer belt 30 is applied to the secondary transferroller 36, thereby generating a transfer electric field at the secondarytransfer nip N2.

The transfer electric field secondarily transfers the full-color tonerimage, specifically, yellow, magenta, cyan, and black toner imagesformed on the intermediate transfer belt 30 onto the sheet P at once.After the secondary transfer of the color toner image from theintermediate transfer belt 30 onto the sheet P, the belt cleaner 35removes residual toner that failed to be transferred onto the sheet Pand therefore remaining on the intermediate transfer belt 30 from theintermediate transfer belt 30. The removed toner is conveyed andcollected into the waste toner container.

Thereafter, the sheet P carrying the full-color toner image is conveyedto the fixing device 20 that fixes the full-color toner image onto thesheet P. Then, the sheet P carrying the fixed full-color toner image isdischarged by the pair of discharge rollers 13 onto the discharge tray14 atop the image forming apparatus 1.

The above describes the image forming operation of the image formingapparatus 1 to form the full-color toner image on the sheet P. Thepresent embodiment is described above with reference to theconfiguration illustrated in FIG. 1, but is not limited thereto. Forexample, alternatively, the image forming apparatus 1 may transferdirectly a toner image from the photoconductors 5 onto the printingmaterial. Alternatively, the image forming apparatus 1 may have a singlecolor development device 7, or have five or more development devices 7.Alternatively, the image forming apparatus 1 may form a bicolor ortricolor toner image by using two or three imaging units 4.

Referring now to FIGS. 3A and 3B, a description is given of thesecondary transfer roller 36.

FIG. 3A is a schematic view of a secondary transfer portion of the imageforming apparatus 1 upon printing. FIG. 3B is a schematic view of thesecondary transfer portion of the image forming apparatus 1 uponmeasurement of a reference pattern.

As illustrated in FIGS. 3A and 3B, the secondary transfer roller 36 iscontactable with and separable from the intermediate transfer belt 30 bya spring 37 and a driver. With this configuration, a test toner imagesuch as a reference pattern described later can be moved to a detectionpoint of the density sensor 50 without being transferred onto theprinting material.

With respect to the exposure device 9, the scanning optical systemincludes optical devices such as a lens, a glass panel, and a mirror,and has different light utilization efficiencies (reflectance ortransmittance) depending on the angle of incidence. The lenses havedifferent degrees of thickness depending on the position of incidence.

The laser light beam Lb polarized by the polygon mirror enters thescanning optical system at an angle of incidence corresponding to anangle of polarization by the polygon mirror. Since the position ofincidence differs depending on the image height, the strength of thelaser light beam Lb on the outer circumferential surface of eachphotoconductor 5 differs depending on the image height. The differencein strength of the laser light beam Lb depending on the image height isreferred to as “shading characteristics”, which is a factor that causesunevenness in density of an output image and thus degrades imagequality. Hence, there are proposed various ways of correcting theshading characteristics.

For example, a plurality of density sensors are used to measure thedensity of a reference pattern (toner patch or toner pattern) formed atdetection points of the plurality of density sensors. According tooutput signals of the plurality of density sensors, the shadingcharacteristics in the main scanning direction are obtained byapproximation with linear interpolation, a linear function, or ahigh-order function to correct a driving signal for an optical scanner.Thus, the shading characteristics are corrected.

Specifically, as illustrated in FIG. 18, five density sensors 50 athrough 50 e are disposed in the main scanning direction of an imagecarrier to measure density of a reference pattern 85 formed by lightemission according to a driving signal uniform in the main scanningdirection. Then, linear or high-order interpolation is performed on fivedensity values outputted from the density sensors 50 a through 50 e tocalculate an estimate output value at a position in the main scanningdirection.

The driving signal for the optical scanner is corrected such that anamount of light emission from the optical scanner is smaller where theestimate output value is higher and larger where the estimate outputvalue is lower. Thus, the image density is equalized in the mainscanning direction.

However, there are problems with such shading correction. For example,since the plurality of density sensors is disposed in the main scanningdirection to increase resolution, production costs are relatively high.Further, an output adjustor is required to equalize output values amongthe plurality of density sensors. In short, processing load isrelatively large.

Furthermore, if there is unevenness in density that cannot beinterpolated by the linear or high-order interpolation, factors of suchunevenness in density cannot be corrected. For example, as illustratedin FIG. 19, if there is irregular unevenness in density betweendetection points of the plurality of density sensors 50 a through 50 e,the factors cannot be detected from the data provided by the pluralityof density sensors 50 a through 50 e. Accordingly, the unevenness indensity may remain in the main scanning direction after correction.

In an embodiment of the present invention, the image forming apparatus 1reliably corrects the shading characteristics in the main scanningdirection with an inexpensive configuration.

Specifically, according to an embodiment of the present invention, atleast one density sensor is driven in the main scanning direction whiledetecting density. Accordingly, upon shading correction, the resolutionin the main scanning direction can be reduced, and therefore, theunevenness in density can be reliably reduced. As described above, usinga plurality of density sensors needs an output adjuster to adjust unevenoutput values of individual sensors, whereas using a single densitysensor obviates the need for such an output adjuster.

Since the density sensor measures density while moving, an output valueof the sensor and a current position of the sensor are correlated.According to an embodiment of the present invention, a driving signalfor an optical scanner is corrected according to data acquired in adensity sensor positional data acquisition process. Thus, the unevennessin density in the main scanning direction can be reliably reduced.

The following describes first through seventh embodiments of the presentinvention in which a density sensor is movably disposed. In thefollowing embodiments, a configuration applicable to the photoconductor5 is also applicable to the intermediate transfer belt 30, and viceversa.

Referring now to FIG. 4, a description is given of a density sensordrive mechanism 500A according to the first embodiment. FIG. 4 is aschematic view of the density sensor drive mechanism 500A. In thepresent embodiment, the photoconductor 5 serves as an image carrier.

The photoconductor 5 is rotatably supported by a pair of side plates 55and 56, and is driven by a drive source. A threaded drive shaft 60 isalso rotatably supported by the pair of side plates 55 and 56 parallelto the axis of the photoconductor 5. The density sensor 50 is attachedto the drive shaft 60.

The density sensor 50 is slidably engaged with a linear guide device toreciprocate in a longitudinal direction of the drive shaft 60 byrotation of the drive shaft 60. The density sensor 50 measures densityof a reference pattern formed on the photoconductor 5 according to ashading correction process described below.

The drive shaft 60 has an end provided with a density sensor drivingunit 69 serving as a driver to drive the density sensor 50. The densitysensor driving unit 69 includes a stepping motor 65 and gears 66 and 67.The gear 66 is attached to an axis of the stepping motor 65. The gear 67is attached to the end of the drive shaft 60.

The gears 66 and 67 engage each other while the stepping motor 65rotates in a forward direction or in a reverse direction. With thisconfiguration, the density sensor 50 reciprocates along the drive shaft60. It is to be noted that a driver to drive the density sensor 50 isnot limited to the density sensor driving unit 69 described above.Alternatively, a belt drive may move the density sensor 50, or a singledirection in which a drive source rotates allows the density sensor 50to reciprocate.

The side plate 55 is provided with a home position mark 57 disposedflush with the outer circumferential surface of the photoconductor 5 inan axial direction thereof. The home position mark 57 serves as a homeposition detector to enhance accurate detection of the position of thedensity sensor 50 in the main scanning direction.

When the density sensor 50 reaches a position where the density sensor50 faces the home position mark 57, and measures a surface of the homeposition mark 57, the density sensor 50 outputs a predetermined value.By the predetermined output value of the density sensor 50, it isdetected that the density sensor 50 reaches the position where thedensity sensor 50 faces the home position mark 57. Thus, a correctreference position of the density sensor 50 can be determined.

In the present embodiment, as illustrated in FIG. 4, the home positionmark 57 is disposed outside the photoconductor 5 in the main scanningdirection. Alternatively, if the photoconductor 5 has a larger widththan an image forming area of the image forming apparatus 1, the homeposition mark 57 may be disposed outside the image forming area withinthe width of the photoconductor 5. Instead of using the home positionmark 57, an optical path shielder may be used to block an optical pathof the density sensor 50. In this case, when the density sensor 50 is atthe reference position, the optical path shielder blocks an optical pathof a transmissive photosensor (i.e. the density sensor 50) to enabledetection of the reference position of the density sensor 50.

Referring now to FIGS. 5A through 5C, a detailed description is given ofthe density sensor 50. Generally, a density sensor that can be used asthe density sensor 50 optically detects density using a light-emittingdevice and a light-receiving device. For example, the density sensor isa reflective photosensor using a light-emitting diode (LED) as thelight-emitting device and a photodiode (PD) as the light-receivingdevice. Alternatively, the density sensor uses a PD and aphototransistor (PTr) as the light-emitting devices.

FIGS. 5A through 5C illustrates different types of such a reflectivephotosensor. FIG. 5A is a schematic view of a density sensor 50A thatincludes a light-emitting device 102A (LED) and a light-receiving device103A in a holder 101A to detect only regular reflection light from areference pattern 85 (toner patch or toner pattern) formed on thephotoconductor 5. FIG. 5B is a schematic view of a density sensor 50Bthat includes a light-emitting device 102B (LED) and a light-receivingdevice 104B in a holder 101B to detect only diffuse reflection lightfrom the reference pattern 85. FIG. 5C is a schematic view of a densitysensor 50C that includes a light-emitting device 102C (LED) andlight-receiving devices 103C and 104C in a holder 101C to detect boththe regular reflection light and the diffuse reflection light from thereference pattern 85.

Such a reflective photosensor is selected as appropriate depending on,e.g., the type of toner used (e.g., black or other colors) and/orconditions of background areas of the photoconductor 5 used. Forexample, if black toner is used, the difference between the regularreflectance from the black toner and regular reflectance from abackground area of the photoconductor 5 is relatively large. In thiscase, the density sensor 50A may be selected. By contrast, if toner ofanother color is used, the difference between the regular reflectancefrom the toner of another color and the regular reflectance from thebackground area of the photoconductor 5 is relatively small. If thedensity sensor 50A is used in this case, the detection accuracy might bedecreased.

Accordingly, if the toner of another color is used, the density sensor50B may be selected. If both the black toner and the toner of anothercolor are used, the density sensor 50B may be selected, or morepreferably, the density sensor 50C may be selected. In the presentembodiment, the density sensor 50 is an optical sensor that detects asurface condition of the photoconductor 5 by a difference betweenincident light and reflection light. The density sensor 50C is employedas the density sensor 50.

The image forming apparatus 1 includes a processor 1000, which isillustrated in FIG. 6. The processor 1000 includes memory devices,namely, a read-only memory (ROM) 1200 and a random access memory (RAM)1300. The ROM 1200 is a storage device that stores a program. Accordingto the program, the processor 1000 acquires density sensor data, andcorrects optical scanning. Acquiring the density sensor data includesacquiring density sensor output data and density sensor positional data.

In the first embodiment, the stepping motor 65 is used as a drive sourceof the density sensor 50. Alternatively, e.g., a DC motor may be used asthe drive source of the density sensor 50. In this case, a rotaryencoder is disposed on the shaft of the DC motor. The processor 1000acquires rotational data of the rotary encoder, instead of therotational data of the stepping motor 65, to acquire the density sensordata (first density sensor positional data process).

Referring now to FIG. 6, a detailed description is given of theprocessor 1000.

FIG. 6 is a block diagram of the processor 1000.

As described above, the image forming apparatus 1 includes the processor1000 serving as a scan controller. The processor 1000 acquires thedensity sensor data including the output data of the density sensor 50and the rotational data of the stepping motor 65 serving as a drivesource, and corrects the optical scanning, specifically, a drivingsignal for the exposure device 9 serving as an optical scanner accordingto the shading correction process described later.

The processor 1000 obtains an equation for correcting light-emittingpower to reduce unevenness in density in the main scanning direction,according to the density sensor data acquired from a reference patternfor reducing the unevenness in density in the main scanning direction,for each imaging unit 4 at each predetermined time. Upon imageformation, the processor 1000 generates a light correction signal usingthe equation for each imaging unit 4 to correct a driving signal foreach light-emitting device 102 of the density sensor 50.

Referring now to FIGS. 7 and 8, a detailed description is given of theshading correction.

Initially with reference to FIG. 7, a description is given of a shadingcorrection process.

FIG. 7 is a flowchart of the shading correction process according to thefirst embodiment. When the shading correction process starts, theexposure device 9 (optical scanner) reads a predetermined referencepattern for shading correction from memories of the image formingapparatus 1, such as the ROM 1200 and the RAM 1300 (S1). Then, theexposure device 9 irradiates each of the photoconductors 5 with thelaser light beam Lb according to the reference pattern thus read to forman electrostatic latent image on each of the photoconductors 5 (S2).Each of the development devices 7 develops the electrostatic latentimage thus formed with toner. Thus, a visible toner image is formed oneach of the photoconductors 5. It is to be noted that the referencepattern is generated in advance according to scan data of a referencewhite board having a predetermined even density.

The reference pattern thus developed is then transferred from each ofthe photoconductors 5 onto the intermediate transfer belt 30. Thus, aunified reference pattern is formed on the intermediate transfer belt30. The reference pattern is moved along with movement of theintermediate transfer belt 30 to a detection point of the density sensor50 (S3). When the reference pattern reaches the detection point, theintermediate transfer belt 30 is stopped so that the density sensor 50can measure the image density of the reference pattern (S4).

The processor 1000 acquires readings that are the measured density datacorrelated with a position of the density sensor 50 in the main scanningdirection calculated from the rotational data of the stepping motor 65(S5). Finally, according to the readings, the processor 1000 calculatesa correction pattern for correcting a driving signal for the exposuredevice 9, as correcting the optical scanning (S6). Imaging process isperformed by optical scanning using the correction pattern. By followingthe above-described process, the driving signal for the exposure device9 is corrected. As a result, the unevenness in density in the mainscanning direction can be reduced.

Referring now to FIGS. 8A through 8C, a description is given of how theshading correction is performed.

FIG. 8A is a plan view of the density sensor drive mechanism 500A withthe reference pattern 85. FIG. 8B is a graph of density sensor output.FIG. 8C is a graph of the optical scanner driving signal.

As illustrated in FIG. 8A, the density sensor 50 continuously measuresthe density of the reference pattern 85 formed on the photoconductor 5while being moved by the density sensor driving unit 69, illustrated inFIG. 4, in the main scanning direction. The processor 1000 acquires thedensity sensor data, specifically, data of the density thus measured bythe density sensor 50.

Simultaneously, the processor 1000 acquires the rotational data of thestepping motor 65 serving as a drive source. A relationship between arotational amount of the stepping motor 65 and a moving amount of thedensity sensor 50 are measured in advance and stored in the ROM 1200.Accordingly, the positional data of the density sensor 50 can be assumedby the rotational data of the stepping motor 65.

Then, the density data and the positional data of the density sensor 50in the main scanning direction are correlated as illustrated in FIG. 8B.From the relationship between the density data and the positional dataof the density sensor 50 in the main scanning direction, the processor1000 corrects the driving signal for the optical scanner 9 (i.e.,optical scanner driving signal) as illustrated in FIG. 8C.

The basis of correction is the same as a comparative correction processillustrated in FIG. 18. However, in the present embodiment, the singledensity sensor 50 measures density while moving in the main scanningdirection, thereby continuously acquiring the density data in the mainscanning direction, unlike the comparative correction illustrated inFIG. 18. Accordingly, in the present embodiment, the density data can beaccurately detected even if local unevenness in density exists betweendensity sensors in the main scanning direction as illustrated in FIG.19.

In addition, using the single density sensor 50 obviates the need for anoutput adjuster that adjusts different output values among a pluralityof density sensors. In other words, it is not necessary to increase thenumber of density sensors according to the resolution. As a result,production costs can be reduced.

Referring now to FIG. 9, a description is given of a problem that can besolved by the second embodiment of the present invention.

FIG. 9 is a layout of a plurality of density sensors 51 through 53 foradjusting image density.

Some image forming apparatuses may have the plurality of density sensors51 through 53 in the main scanning direction as illustrated in FIG. 9because installation errors of machine and/or configuration of adevelopment device may cause unevenness in density in the main scanningdirection even under the same image forming conditions. The unevennessin density may be caused by deterioration of the machine and/or changesin toner stability over time and/or due to the environment. To correctthe unevenness in density, a predetermined reference pattern is formedon the photoconductor 5 and the density sensors 51 through 53 (sensorsfor adjusting image density) measure the density of the referencepattern to adjust the image forming conditions such as toner density anddevelopment bias.

Generally, each of the density sensors 51 through 53 may have any one ofthe configurations illustrated in FIGS. 5A through 5C. In other words,the density sensors 51 through 53 for adjusting image density have thesame configuration as the density sensor 50. In such a case, the densityof the reference pattern can be measured across the main scanningdirection by moving the density sensor 50 in the configurationillustrated in FIG. 4. In other words, the three density sensors 51through 53 of FIG. 9 can be unified into a single sensor.

On the other hand, some image forming apparatuses may not be able tofine-tune the image forming conditions in the main scanning direction.For example, such image forming apparatuses may uniformly correct theimage forming conditions in the main scanning direction, such that anaverage value of density measured by a plurality of sensors becomes atarget density value. In such a way of correction of the image formingconditions, sufficient data may be acquired by measuring severalpositions as illustrated in FIG. 9.

In such a case, if the three density sensors 51 through 53 of FIG. 9 areunified into a single sensor and are driven in the configurationillustrated in FIG. 4, a measuring time may be longer than a measuringtime taken when the plurality of sensors 51 through 53 are used asillustrated in FIG. 9, because the speed at which a density sensordriving unit drives a density sensor is limited.

Generally, image forming conditions are corrected more frequently thanthe shading is corrected. Accordingly, any increase in the measuringtime may generate a “standby time” during which image formation cannotbe performed. For mass high-speed printing, the standby time may be abigger problem than production costs.

Referring now to FIG. 10, a description is given of a density sensordrive mechanism 500B according to the second embodiment.

FIG. 10 is a schematic view of the density sensor drive mechanism 500B.

For mass high-speed printing, the image forming apparatus 1 may have thedensity sensor drive mechanism 500B. In FIG. 10, three density sensors51B through 53B are disposed over the width of the intermediate transferbelt 30 serving as an image carrier. Specifically, one movable densitysensor 51B and two fixed density sensors 52B and 53B are disposed.

The movable density sensor 51B is movable in the main scanning directionby a drive shaft 60B serving as a density sensor driver. By contrast,the fixed density sensors 52B and 53B are attached to a support member70 fixed across the width of the intermediate transfer belt 30.

The drive shaft 60B is the same as the drive shaft 60 of FIG. 4. In FIG.10, the motor 65 and the gears 66 and 67 are omitted for ease ofillustration.

In the second embodiment, the movable density sensor 51B is positionedaway from the fixed density sensors 52B and 53B in the sub-scanningdirection so as not to interrupt the fixed density sensors 52B and 53Bwhile moving. Alternatively, for example, the movable density sensor 51Bmay be separated from the intermediate transfer belt 30 in a heightdirection, as long as the movable density sensor 51B can measure theouter circumferential surface of the intermediate transfer belt 30.

In such a configuration, if the movable density sensor 51B is fixed at aposition illustrated in FIG. 10 upon measurement that obviates detailedresolution in the main scanning direction for, e.g., correcting theimage forming conditions, the measurement can be completed in a shortertime. By contrast, upon measurement that requires detailed resolution inthe main scanning direction for, e.g., shading correction, the movabledensity sensor 51B measures density while moving for the correction asillustrated in FIGS. 8A through 8C.

Although the configuration of FIG. 10 has the plurality of densitysensors 51B through 53B and therefore production costs increase, theimage forming conditions can be corrected without increasing the standbytime. In addition, the unevenness in density in the main scanningdirection due to the shading characteristics can be reliably reduced. Itis to be noted that the number of density sensors is not limited tothree, but can be any number.

Referring now to FIG. 11, a description is given of a density sensordrive mechanism 500C according to the third embodiment.

FIG. 11 is a schematic view of the density sensor drive mechanism 500C.

Unlike the second embodiment, the density sensor drive mechanism 500Cdrives a plurality of movable density sensors. For example, asillustrated in FIG. 11, density sensors 51C and 52C are movable. In thiscase, the density sensor 53C is a fixed sensor. The movable densitysensors 51C and 52C are attached to drive shafts 61 and 62,respectively. The drive shafts 61 and 62 are threaded, and serve asdensity sensor drivers.

The drive shaft 61 has an end provided with a gear 67. Similarly, thedrive shaft 62 has an end provided with a gear 68. Engaged with thegears 67 and 68 is a gear 66 that is attached to an axis of a steppingmotor 65C. Accordingly, the drive shaft 61 and 62 are driven by thestepping motor 65C as a common drive source via the gears 66 through 68.The drive shafts 61 and 62 are threaded in opposite directions.Therefore, the movable density sensors 51C and 52C move in the oppositedirections when the stepping motor 65 rotates.

By increasing the number of movable density sensors as in the presentembodiment, two reference patterns can be measured by one driving uponshading correction. For example, in an image forming apparatus havingdevelopment devices for a plurality of colors, reference patterns havingdifferent colors may be formed at detection points of the movabledensity sensors 51C and 52C. In this case, two types of color data canbe acquired at once, thereby shortening the measuring time.

As described above, the drive shafts 61 and 62 serving as density sensordrivers are threaded in opposite directions. Alternatively, a singledirection in which the drive source rotates allows the movable densitysensors 51C and 52C to reciprocate. In such a configuration, althoughthe direction in which the drive source rotates is constant, the movabledensity sensors 51C and 52C move in opposite directions.

Referring now to FIG. 12, a description is given of a density sensordrive mechanism 500D according to the fourth embodiment.

FIG. 12 is a schematic view of the density sensor drive mechanism 500D.

Instead of using gears, an endless belt 81 is used as a density sensordriver. In the fourth embodiment, the density sensor drive mechanism500D drives two movable density sensors 51D and 52D and one fixeddensity sensor 53D. Like the second and third embodiments, the fixeddensity sensor 53D is attached to a center of a support member 70D.

The movable density sensors 51D and 52D are slidably disposed on guiderails 71 and 72, respectively, and connected to the endless belt 81driven by a stepping motor 80 serving as a drive source.

The endless belt 81 is stretched around a plurality of guide rollers 82along the guide rails 71 and 72. As in the fourth embodiment, thedensity sensor driver is not limited to the threaded drive shafts 60through 62, but can by any driver.

Referring now to FIGS. 13A and 13B, a description is given of a densitysensor drive mechanism 500E according to the fifth embodiment.

FIG. 13A is a plan view of the density sensor drive mechanism 500E. FIG.13B is a graph of density sensor output.

The density sensor drive mechanism 500E drives a movable density sensor50E. In the first embodiment, the positional data of the movable densitysensor 50 is acquired from the rotational data of the stepping motor 65serving as a drive source in the first density sensor positional dataprocess. However, in the fifth embodiment, the processor 1000 acquirespositional data of the movable density sensor 50E in a second densitysensor positional data acquisition process, in which the processor 1000calculates a position of the density sensor 50E in the main scanningdirection according to detected data of an index part 90 b provided bythe density sensor 50E. As illustrated in FIG. 13A, a reference pattern90 includes an imaged part 90 a and the index part 90 b. With the indexpart 90 b, the reference pattern 90 exhibits intermittent changes indetection density. In short, the processor 1000 acquires the positionaldata of the movable density sensor 50E in the second density sensorpositional data acquisition process using the detection data of theindex part 90 b of the reference pattern 90, instead of the rotationaldata of a motor such as the stepping motor 65.

In the present embodiment, the reference pattern 90 is a discontinuouspattern having a blank area as the index part 90 b between the imagedparts 90 a. When the reference pattern 90 is used, output values of themovable sensor 50E acquired while the movable sensor 50E is movingsignificantly drops at each index part 90 b as illustrated in FIG. 13B.

A threshold output value of the movable density sensor 50E at a boundarybetween the imaged part 90 a and the index part 90 b is measured inadvance and stored in the ROM 1200 of the processor 1000 illustrated inFIG. 6. By measuring the number of output values of the movable densitysensor 50E lower than the threshold during detection, it can bedetermined which imaged part 90 a the movable density sensor 50Ecurrently measures in the main scanning direction.

Such a configuration obviates the need for rotational data of thestepping motor 65 or 80. Accordingly, e.g., a DC motor can be used as adrive source of a drive shaft 60E, instead of a stepping motor such asthe stepping motor 65 or 80 that is generally more expensive than a DCmotor. For moving the movable density sensor 50E to a referenceposition, the reference pattern 90 formed at a scannable position on theintermediate transfer belt 30 may obviate the need for the home positionmark 57.

Accordingly, in the fifth embodiment, the shading correction can beperformed with a less expensive configuration than that of the firstembodiment. In addition, since the index part 90 b is a blank area inwhich an image is not formed, the amount of toner used can be reduced.

The index part 90 b is formed by the processor 1000 of FIG. 6. Theprocessor 1000 determines the number of the index parts 90 b in a firstreference pattern formation process, according to, e.g., the resolutionof density data required for the shading correction in the main scanningdirection. The index parts 90B is formed, e.g., at equal intervals.

Referring now to FIGS. 14 and 15, a description is given of a densitysensor drive mechanism 500F according to the sixth embodiment.

FIG. 14 is a plan view of the density sensor drive mechanism 500F. Whenusing the reference pattern 85 of FIG. 8A, the photoconductor 5 servingas an image carrier stops moving in the sub-scanning direction while themovable sensor 50 is moving in the main scanning direction. Similarly,when using the reference pattern 90 of FIG. 13A, the intermediatetransfer belt 30 serving as an image carrier stops moving in thesub-scanning direction while the movable sensor 50E is moving in themain scanning direction. In short, stopping and resuming movement of theimage carrier are extra processes compared to the comparative shadingcorrection illustrated in FIG. 18.

Hence, in the sixth embodiment, a reference pattern 91 for shadingcorrection is changed according to a relationship between a speed atwhich the intermediate transfer belt 30 serving as an image carrierrotates in the sub-scanning direction and a speed at which a movablesensor 50F moves in the main scanning direction. The processor 1000performs a second reference pattern formation process for changing thereference pattern according to a program stored in the ROM 1200 and withthe CPU 1100 illustrated in FIG. 6. The CPU 1100 receives the speed atwhich the intermediate transfer belt 30 rotates in the sub-scanningdirection and the speed at which the movable sensor 50F moves in themain scanning direction.

As illustrated in FIG. 14, in the second reference pattern formationprocess, the processor 1000 forms the reference pattern 91 on theintermediate transfer belt 30 in an oblique direction to the mainscanning direction at a predetermined angle θ, such that the referencepattern 91 is constantly at a measurable position of the movable densitysensor 50F attached to a drive shaft 60F even while the intermediatetransfer belt 30 is moving in the sub-scanning direction.

Accordingly, the shading correction can be performed while continuouslymoving the intermediate transfer belt 30 serving as an image carrier inthe main scanning direction, instead of stopping the intermediatetransfer belt 30. Alternatively, the reference pattern 91 may be adiscontinuous reference pattern having an index part such as the indexpart 90 b or an imaged area at a predetermined position.

FIG. 15 is a flowchart of a shading correction process according to thesixth embodiment.

When forming the oblique reference pattern 91 of FIG. 14, the shadingcorrection process can be performed as illustrated in FIG. 15.

When a shading correction process starts, the processor 1000 acquiresspeed data of the image carrier (i.e., intermediate transfer belt 30)(S11) and speed data of the movable density sensor 50F (S12). The speeddata of the image carrier and the speed data of the movable densitysensor 50F are usually fixed, and therefore, are stored in advance inthe ROM 1200 and the RAM 1300 of the image forming apparatus 1.

It is to be noted that, in image forming apparatuses having variableimage carrier speed, the speed data is stored in the RAM 1300 every timewhen the speed is changed such that the latest speed data can beacquired.

Then, the processor 1000 calculates the reference pattern to form (i.e.,reference pattern 91) according to the speed data of the image carrierand the speed data of the movable density sensor 50F thus acquired(S13). Subsequent steps S14 through S18 are the same as steps S2 throughS6 of FIG. 7.

Referring now to FIGS. 16, 17A and 17B, a description is given of ashading correction process according to the seventh embodiment.

FIG. 16 is a plan view of the intermediate transfer belt 30 serving asan image carrier, illustrating positions to detect density according tothe seventh embodiment.

When forming the reference pattern 91 of FIG. 14, the image carrier isnot stopped while measuring. Hence, in the seventh embodiment, theshading correction is performed using a test area 93, during printing ofa printing area 92 performed by the image forming apparatus 1.

Alternatively, during printing, a density sensor (e.g., density sensor50) may be moved in the main scanning direction while the shadingcorrection is performed, and the density sensor may be moved back to aninitial reference position in the main scanning direction. As a result,the standby time can be shortened.

When the shading correction is performed using the test area 93, theprocesses of FIGS. 17A and 17B are followed. Specifically, FIG. 17A isan overall flowchart of a print job including determination of whetheror not to perform a shading correction process according to the seventhembodiment. FIG. 17B is a flowchart of the shading correction processillustrated in FIG. 17A.

Referring to FIG. 17A, when a print job starts (S21), it is determinedwhether a shading correction process is required (S22). It may bedetermined that shading correction is required when, e.g., 1) the imageforming apparatus 1 is activated, 2) the temperature and/or humidityhave changed by a predetermined amount since the last level, 3) apredetermined time has elapsed after the last shading correction, 4) apredetermined number of sheets have been printed after the last shadingcorrection; or 5) the shading correction is instructed.

Conditions 1 through 5 described above are stored in the ROM 1200 andthe RAM 1300. It is determined whether the shading correction process isrequired according to a program run by the CPU 1100. The conditions suchas the above-described conditions 1 through 5 may be any conditionderived from the operating environment of the image forming apparatus 1.If it is determined that the shading correction process is not required(NO in S22), then the print job proceeds (S24) and steps S22 through S24are repeated until it is determined that the print job is completed (YESin S25).

On the other hand, if it is determined that the shading correctionprocess is required (YES in S22), then the shading correction process isperformed (S23). The shading correction process is illustrated in FIG.17B in detail, and is substantially the same as the process of FIG. 15,except that it is determined whether or not a subject area is the testarea 93 (S34) before a reference pattern is formed (S35), at which point(YES in S34), the subsequent steps are performed from step S35.

The present invention has been described above with reference tospecific exemplary embodiments. It is to be noted that the presentinvention is not limited to the details of the embodiments describedabove, but various modifications and enhancements are possible withoutdeparting from the scope of the invention. It is therefore to beunderstood that the present invention may be practiced otherwise than asspecifically described herein. For example, elements and/or features ofdifferent illustrative exemplary embodiments may be combined with eachother and/or substituted for each other within the scope of thisinvention. The number of constituent elements and their locations,shapes, and so forth are not limited to any of the structure forperforming the methodology illustrated in the drawings.

What is claimed is:
 1. An image forming apparatus comprising: aphotoconductor; an optical scanner, including a light source to emitlight, to irradiate a surface of the photoconductor in a main scanningdirection with the light to form a latent image on the surface of thephotoconductor; a development device to develop the latent image into atoner image; a movable first density sensor to detect unevenness indensity of the toner image in the main scanning direction; a densitysensor driver to move the first density sensor in the main scanningdirection; and a processor to correct a driving signal for the lightsource according to image data to reduce the unevenness in density inthe main scanning direction, according to positional data of the firstdensity sensor in the main scanning direction and an output value of thefirst density sensor.
 2. The image forming apparatus according to claim1, wherein the processor forms a reference pattern for shadingcorrection having a predetermined density detectable by the firstdensity sensor on the photoconductor.
 3. The image forming apparatusaccording to claim 2, further comprising a home position detectordisposed at a predetermined position in the main scanning direction ofthe photoconductor to detect a home position of the first density sensoras a reference for the positional data of the first density sensor inthe main scanning direction.
 4. The image forming apparatus according toclaim 3, wherein the processor performs a first density sensorpositional data acquisition process to calculate a position of the firstdensity sensor in the main scanning direction according to data providedby the density sensor driver.
 5. The image forming apparatus accordingto claim 1, wherein the processor forms a reference pattern having anindex part at a predetermined position in the main scanning directionand exhibiting intermittent changes in detection density at the indexpart, and wherein the processor performs a second density sensorpositional data acquisition process to calculate a position of the firstdensity sensor in the main scanning direction according to detectiondata of the index part provided by the first density sensor.
 6. Theimage forming apparatus according to claim 5, wherein the processorperforms a first reference pattern formation process to form thereference pattern having the index part according to a requiredresolution in the main scanning direction.
 7. The image formingapparatus according to claim 2, wherein the processor performs a secondreference pattern formation process to form the reference pattern on thephotoconductor in an oblique direction to the main scanning direction ata predetermined angle, such that the reference pattern is detectableregardless of a position of the first density sensor.
 8. The imageforming apparatus according to claim 7, wherein the processor performsthe second reference pattern formation process according to a speed atwhich the photoconductor rotates in a sub-scanning direction and a speedat which the first density sensor moves in the main scanning direction.9. The image forming apparatus according to claim 2, wherein theprocessor forms the reference pattern in a test area between printingareas arranged in a sub-scanning direction of the photoconductor, anddetects the reference pattern with the first density sensor while thephotoconductor is rotating.
 10. The image forming apparatus according toclaim 1, further comprising a transfer device to transfer the tonerimage from the photoconductor to a transfer belt, wherein the processorforms a reference pattern for shading correction having a predetermineddensity detectable by the first density sensor on the transfer belt. 11.The image forming apparatus according to claim 10, further comprising ahome position detector disposed at a predetermined position in the mainscanning direction of the transfer belt to detect a home position of thefirst density sensor as a reference for the positional data of the firstdensity sensor in the main scanning direction.
 12. The image formingapparatus according to claim 11, wherein the processor performs a firstdensity sensor positional data acquisition process to calculate aposition of the first density sensor in the main scanning directionaccording to data provided by the density sensor driver.
 13. The imageforming apparatus according to claim 10, wherein the processor performsa second reference pattern formation process to form the referencepattern on the transfer belt in an oblique direction to the mainscanning direction at a predetermined angle, such that the referencepattern is detectable regardless of a position of the first densitysensor.
 14. The image forming apparatus according to claim 13, whereinthe processor performs the second reference pattern formation processaccording to a speed at which the transfer belt rotates in asub-scanning direction and a speed at which the first density sensormoves in the main scanning direction.
 15. The image forming apparatusaccording to claim 10, wherein the processor forms the reference patternin a test area between printing areas arranged in a sub-scanningdirection of the transfer belt, and detects the reference pattern withthe first density sensor while the transfer belt is rotating.
 16. Theimage forming apparatus according to claim 1, wherein the first densitysensor doubles as a sensor for adjusting image density.
 17. The imageforming apparatus according to claim 1, further comprising a fixedsecond density sensor for adjusting image density.
 18. The image formingapparatus according to claim 1, wherein the density sensor driver movesthe first density sensor to a reference position in the main scanningdirection during image printing.